Results of torsion tests on solid circular and rectangular bars for the determination of the shear response of unidirectional graphite/epoxy composites are reported. Such tests have been used by some investigators for determining the shear modulus. Determination of nonlinear shear response from torque-twist (or shear strain) data is, however, a difficult task. It is a common practice to determine the shear stress distribution by the use of linear elasticity theory even when the shear response is highly nonlinear.

In this study, the inelastic stress states in solid circular and rectangular bars are considered. A simple procedure is used for analyzing the data for circular bars. For rectangular specimens it is assumed that the material is elastoplastic, showing strain hardening such that the shear stress-strain response is of the Ramberg-Osgood type. A method is suggested for backing out the RambergOsgood parameters from test data for rectangular bars. Representative data from tests on both types of bars are compared with data from other tests. The results compare well with Iosipescu and two-rail shear data up to a shear strain level of 3%. Responses obtained from specimens of different cross-section or dimensions and lengths are presented. Failure mechanisms and reasons for low ultimate strains (as compared to Iosipescu and two-rail shear tests) are discussed. Based on analytical studies of end effects and observed failure mechanisms, suggestions are made regarding choice of specimen dimensions for obtaining reliable data. It appears that torsion tests provide a useful simple method for determining nonlinear shear response of unidirectional composites, especially when other methods may not be suitable due to special types of product forms like pultruded bars or thick sections.